Wireless communication devices such as laptops, cell phones and other devices may use technology such as Worldwide Interoperability for Microwave Access (WiMAX) to enable Internet or other types of wireless communications. WiMAX is a telecommunications technology aimed at providing wireless data over long distances in a variety of ways, from point-to-point links to full mobile cellular type access. Wireless communication devices typically use a technique such as Orthogonal Frequency Division Multiplexing (OFDM), one version of which is Orthogonal Frequency Division Multiple Access (OFDMA), to modulate a communication signal for wireless communication. The principles of OFDM modulation have been in existence for several decades. In recent years, OFDM techniques have been employed in data delivery systems over the phone line, digital radio and television, and wireless networking systems. OFDM spread spectrum techniques distribute data over a large number of carriers that are spaced apart at precise frequencies. OFDM works generally by dividing one high-speed data carrier into multiple low speed sub-carriers which are used for transmission of data in parallel. The sub-carriers are typically the smallest practical allotment of a frequency range in a bandwidth for transmission purposes. The sub-carriers can be assigned to one or more sub-carrier sets. Put another way, the data stream of interest is divided into multiple parallel bit streams known as symbols, each symbol transmitted over a different sub-carrier at a lower effective bit rate. Before final power amplification and transmission, the multi-carrier OFDM symbols are converted into the time domain using Inverse Fast Fourier Transform techniques resulting in a relatively high speed time domain signal with a large peak to average ratio (PAR).
An OFDM preamble symbol is developed to precede a data OFDM symbol. The preamble symbol, also known as a preamble sequence, is a binary sequence that consists of 0s and 1s, and is associated with sub-carrier signals from the set of sub-carriers. A preamble symbol is generated by a base station, which is located in a segment of base stations and has a segment number, using a process in which one of several candidate preamble sequences is chosen based on the base station's IDCell number and segment number. The OFDM communication system uses a preamble sequence for all frame timing synchronization, frequency synchronization, and channel estimation. The OFDM communication system may perform frame timing synchronization, frequency synchronization, and channel estimation using a guard interval and a pilot sub-carrier in addition to the preamble symbol. The preamble sequence is used to transmit known symbols at a beginning part of every frame or data burst, and update estimated time/frequency/channel information at a data transmission part, using information on the guard interval and the pilot sub-carrier.
Once a preamble sequence is chosen, the base station modulates sub-carriers with the chosen preamble sequence, performs IFFT (Inverse Fast Fourier Transform), and adds a cyclic prefix to reduce the inter-symbol interface and inter-channel interface introduced by the multi-path channel through which the signal is propagated.
IEEE 802.11a/g compliant transmission systems achieve high data transmission rates using OFDM encoded symbols mapped to up to 64 QAM (Quadrature Amplitude Modulation) multi-carrier constellations and beyond. In the case of 802.11a/g, there are up to 52 defined sub-carriers or tones, of which 48 sub-carriers or tones are available to carry data (the four remaining are pilot sub-carriers or tones, which carry predetermined data). These sub-carriers are substantially orthogonal to one another, so they can be spaced closer together than in conventional frequency division multiplexing. Mathematically, the integral of the product of any two orthogonal sub-carriers is zero. This property allows the separation of sub-carriers at the receiver without inference from other sub-carriers.
Signal power is estimated in developing RSSI (Receive Signal Strength Indicator) and PCINR (Physical Carrier to Interference and Noise Ratio) for OFDM systems.
RSSI, measured in dBm, is a signal output indicator that is proportional with RF input power. As a result, RSSI signal output can be used to measure RF signal strength on a specific channel and indicate the usage on that channel. RSSI measures the total received power of the preamble of the desired base station. PCINR measures the ratio of average desired signal power to average residual error such as noise and interference. RSSI is used mainly for handover between base stations that have IDCell numbers and segments, and PCINR is used mainly for burst adaptive profile selection and handover between base stations.
Typically, signal power is estimated using either the preamble symbol or the regular OFDM data symbol. One method for estimating signal power using the preamble symbol involves estimating the power of the received signal that corresponds to each preamble sequence. However, typically, estimating received signal power requires estimating the channel gain from the base station for each preamble sequence. Estimating the channel gain creates high level complexity and, therefore, reduces system performance.